1. Field of the Invention
The present invention relates to a fundus camera that performs fundus photographing by using a plurality of light-emitting diode (LED) elements as photographing light sources.
2. Description of the Related Art
In a conventional non-mydriatic fundus camera used for a group medical examination or the like, in order to prevent miosis of a subject, a visible light cut filter is inserted before a halogen light source and only an infrared component of the halogen light source is used as observation light to illuminate the subject. Further, visible light emitted from a xenon light source is used for photographing.
With respect to light with which a fundus is illuminated, it is necessary to separate fundus illumination light and photographing light from the fundus, in order to prevent light reflected on a cornea or a crystalline lens of the subject's eye from entering into a photographic diaphragm. Thus, an illumination optical system includes a diaphragm having a ring-shaped aperture disposed in a position roughly optically conjugate with the cornea or the crystalline lens. Similarly, in positions roughly optically conjugate with a pupil, a photographic optical system includes a diaphragm having a circular aperture, and the illumination optical system includes a pupil stop having a ring-shaped aperture.
Conventionally, photographing is performed by using light emitted from a xenon tube. Recently, however, for the purpose of miniaturizing an electrical device unit, saving energy, reducing costs, and preventing heat generation of a light source unit, it has been discussed to use light sources other than the halogen light source. An LED element of high luminance that emits a white light is one of major candidates of the light source, and has been discussed as a photographing light source used in the fundus camera.
As an example, Japanese Patent Application Laid-Open No. 2007-29726 discusses a technique for using LED elements in fundus illumination. According to the technique discussed in Japanese Patent Application Laid-Open No. 2007-29726, a plurality of LED elements are arranged into a ring shape to form a ring light source, and a diameter of the ring light is variable. The diameter of the ring light is made variable in order to prevent illumination scattering light generated at an anterior eye of a subject's eye from mixing into a photographed image, and to optimize brightness and a contrast of the photographed image.
Japanese Patent Application Laid-Open No. 2006-174984 discusses a technique for uniformly illuminating a fundus by diffusing and reflecting lights emitted from a plurality of arrayed LED elements, and enabling use of various fundus photographing methods by using LED elements which can emit light with a plurality of luminescent colors.
Generally, in order to obtain a high quality image, in other words, a high contrast fundus image, the fundus needs to be uniformly illuminated with the illumination light. Thus, the light emitted from the LED element is not directly guided to the fundus to illuminate it, but diffused to remove directional characteristics.
However, the conventional xenon tube used as the photographing light source cannot control ring-shaped light amount distribution. On the other hand, the plurality of LED elements is discretely arranged into the ring shape to form an illumination ring. Hence, while each LED element can be controlled, the light amount distribution is not actively controlled by controlling the each LED element.